PRODUCTION METHOD OF 225Ac

ABSTRACT

One embodiment of the present invention relates to a production method of 225Ac includes; a production step of a 226Ra target including an electrodeposition step of electrodepositing a 226Ra-containing substance on a substrate by using an electrodeposition solution that contains 226Ra ions and a pH buffer, and an irradiating step of irradiating the 226Ra target with at least one selected from charged particles, photons, and neutrons.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/619,275, filed on Dec. 15, 2021, which is a national stageapplication of PCT/JP2020/023971, filed on Jun. 18, 2020, and whichclaims priority to Japanese Patent Application No. 2019-113698, filed onJun. 19, 2019, the entire contents of all of which are herebyincorporated by reference.

TECHNICAL FIELD

One embodiment of the present invention relates to a production methodof a ²²⁶Ra target, a production method of ²²⁵Ac, or an electrodepositionsolution for producing a ²²⁶Ra target.

BACKGROUND ART

²²⁵Ac, which is one of alpha-radionuclides, is a radionuclide having ahalf-life of 10 days, and, in recent years, there has been a growingexpectation that ²²⁵Ac will be used as a therapeutic nuclide fortreating, for example, cancer.

²²⁵Ac is produced through a (p, 2n) nuclear reaction that involves, forexample, irradiating a ²²⁶Ra target with protons using an accelerator.

As a production method of such a ²²⁶Ra target, there is known a methodfor electrodepositing a ²²⁶Ra-containing substance on an aluminumsurface by using an isopropanol-containing plating solution (see PatentLiterature 1).

CITATION LIST Patent Literature

-   Patent Literature 1: JP-A-2007-508531

SUMMARY OF INVENTION

However, according to existing ²²⁶Ra electrodeposition methods, theelectrodeposition solution undergoes a decrease in electricalconductivity, and high voltage needs to be applied in order toelectrodeposit a predetermined amount of ²²⁶Ra. This increases the sizeof, for example, power supplies and equipment, and a cooling step forremoving generated heat may become necessary in some cases. Moreover, ithas been found that despite application of high voltage, ²²⁶Ra ionscontained in the electrodeposition solution cannot be efficientlydeposited on a substrate.

An embodiment of the present invention provides a production method of a²²⁶Ra target, the method capable of efficiently electrodepositing ²²⁶Raions contained in an electrodeposition solution on a substrate.

The present inventors have conducted extensive investigations on themethod for addressing the above issues, and found that the above issuescan be addressed by a particular production method, thereby completingthe present invention.

One aspect of the present invention provides a production method of a²²⁶Ra target, including an electrodeposition step of electrodepositing a²²⁶Ra-containing substance on a substrate by using an electrodepositionsolution that contains ²²⁶Ra ions and a pH buffer.

In addition, another aspect of the present invention provides aproduction method of ²²⁵Ac, the method including an irradiating step ofirradiating a ²²⁶Ra target, which has been produced by the aboveproduction method of a ²²⁶Ra target, with at least one selected fromcharged a particle, a photon, and a neutron.

Furthermore, yet another aspect of the present invention provides anelectrodeposition solution for producing a ²²⁶Ra target, containing²²⁶Ra ions and a pH buffer, and the electrodeposition solution issubstantially free of alcohols.

According to an embodiment of the present invention, ²²⁶Ra ionscontained in the electrodeposition solution can be efficientlyelectrodeposited on a substrate without applying high voltage. Thus,according to an embodiment of the present invention, the size of thefacility for producing ²²⁶Ra targets can be reduced, and ²²⁶Ra targetscan be produced without performing a cooling step. In other words,according to one embodiment of the present invention, ²²⁶Ra targets canbe produced with less space and less energy, and by a simple method.

According to an embodiment of the present invention, since a ²²⁶Ratarget that contains a predetermined amount of a ²²⁶Ra-containingsubstance can be produced, a predetermined amount of ²²⁵Ac can be easilyproduced by using this target with less space and less energy.

DESCRIPTION OF EMBODIMENTS [Production Method of ²²⁶Ra Target]

A production method of a ²²⁶Ra target according to one embodiment of thepresent invention (hereinafter, this method may also be referred to as a“present production method”) includes an electrodeposition step ofelectrodepositing a ²²⁶Ra-containing substance on a substrate by usingan electrodeposition solution that contains ²²⁶Ra ions and a pH buffer.

According to the present production method, the ²²⁶Ra-containingsubstance is electrodeposited on a substrate. Examples of the²²⁶Ra-containing substance include ²²⁶Ra metal and ²²⁶Ra salts. That is,the ²²⁶Ra target obtained through the present production method contains²²⁶Ra metal or a ²²⁶Ra salt.

<Electrodeposition Solution>

The electrodeposition solution is not particularly limited and may beany liquid that contains ²²⁶Ra ions and a pH buffer, and may furthercontain components other than these, if necessary.

In view of, enhancing the effects of the present invention, theelectrodeposition solution is preferably an aqueous solution. In thiscase, pure water or ultrapure water is preferably used.

In the present production method, although two or more electrodepositionsolutions may be used, one electrodeposition solution is usually used.

In the above existing method for electrodepositing a ²²⁶Ra-containingsubstance, an alcohol such as isopropanol was used.

However, investigations conducted by the present inventors have foundthat, according to the present production method, a ²²⁶Ra-containingsubstance can be electrodeposited on a substrate without using analcohol. Thus, from the viewpoints such as that the decrease inelectrical conductivity of the electrodeposition solution can besuppressed and that ²²⁶Ra ions contained in the electrodepositionsolution can be efficiently electrodeposited on a substrate, theelectrodeposition solution is preferably substantially free of alcohols.

Examples of the alcohol include C1-C5 alkyl alcohols such as ethanol,1-propanol, and isopropanol.

In addition, the electrodeposition solution is preferably substantiallyfree of acetones for the same reasons as for the alcohols.

Here, the meaning of substantially free of alcohols or acetones is thatalcohols or acetones are not intentionally added to theelectrodeposition solution. Specifically, the alcohol or acetone contentin the electrodeposition solution is preferably 0.01 mass % or less, andthe lower limit of the content is 0 mass %.

From the viewpoints such as that ²²⁶Ra ions can be more efficientlyelectrodeposited on a substrate, the electrodeposition solutionpreferably contains carboxylate ions (COO⁻) and more preferably containsacetate ions.

From the viewpoints such as that ²²⁶Ra ions can be more efficientlyelectrodeposited on a substrate, the electrodeposition solution ispreferably acidic at the start of the electrodeposition step, and the pHof the electrodeposition solution in this case is preferably 4 or moreand more preferably 5 to 6. The pH of the electrodeposition solutionduring (in the middle of) the electrodeposition step is preferably 4 to9 and more preferably 6 to 8. The pH of the prepared electrodepositionsolution may be measured by using, for example, a pH meter or a pH-testpaper; however, the pH is preferably calculated from, for example, thetypes of the raw materials blended in the electrodeposition solution andthe amounts thereof used, and is preferably adjusted by, for example,the types of the raw materials blended in the electrodeposition solutionand the amounts thereof used.

<<Acid>>

The electrodeposition solution is preferably prepared by using an acid.

Although the acid is not particularly limited, from the viewpoints suchas that ²²⁶Ra ions can be more efficiently electrodeposited on asubstrate, the acid preferably has no chelating effect on the ²²⁶Raions.

One acid may be used alone, or two or more acids may be used.

Examples of the acid include inorganic acids and carboxylic acids having2 to 6 carbon atoms. Examples of the inorganic acids include nitricacid, hydrochloric acid, and boric acid. Examples of the carboxylicacids having 2 to 6 carbon atoms include acetic acid, succinic acid, andbenzoic acid.

From the viewpoint of, for example, improving the yield of ²²⁵Ac, theacid is preferably a monovalent or divalent acid.

The acid concentration in the electrodeposition solution may beappropriately selected according to the type of the acid used, and theacid is preferably used such that the electrodeposition solution isacidic at the start of the electrodeposition step. The specificconcentration is preferably 0.005 to 0.2 mol/L and more preferably 0.005to 0.05 mol/L. When the acid concentration is within this range, ²²⁶Raions can be more efficiently electrodeposited on a substrate.

For the same reason, especially when hydrochloric acid is used as theacid, the concentration thereof in the electrodeposition solution ispreferably 0.04 mol/L or less, and more preferably 0.005 to 0.035 mol/L;and when nitric acid is used as the acid, the concentration thereof inthe electrodeposition solution is preferably 0.2 mol/L or less and morepreferably 0.005 to 0.1 mol/L.

When acetic acid is used as the acid, the concentration thereof in theelectrodeposition solution is preferably 0.2 mol/L or less and morepreferably 0.05 to 0.1 mol/L.

The amount of the acid used relative to 0.02 mol/L of ²²⁶Ra ions ispreferably 0.5 mol/L or less and more preferably 0.001 to 0.4 mol/L.

According to the present production method, even when such an amount ofthe acid is used, ²²⁶Ra ions can be more efficiently electrodeposited ona substrate.

<<pH Buffer>>

The pH buffer is not particularly limited as long as rapid changes in pHcan be prevented; however, a pH buffer that can maintain the pH of theelectrodeposition solution to about 4 to 9 and preferably about 6 to 8during (in the middle of) the electrodeposition step is preferably used.

Although the pH buffer is not particularly limited, a pH buffer solutionis usually used.

One pH buffer or two or more pH buffers may be used in theelectrodeposition solution.

Examples of the pH buffer include ammonium chloride; carbonate saltssuch as ammonium carbonate, sodium carbonate, potassium carbonate,calcium carbonate, and magnesium carbonate; hydrogen carbonate saltssuch as ammonium hydrogen carbonate, sodium hydrogen carbonate, andpotassium hydrogen carbonate; acetate salts such as ammonium acetate,sodium acetate, and potassium acetate; succinate salts such asmonosodium succinate, disodium succinate, monopotassium succinate,dipotassium succinate, monoammonium succinate, and diammonium succinate,and benzoate salts such as sodium benzoate, potassium benzoate, andammonium benzoate. Among these, from the viewpoints such as that the pHof the electrodeposition solution can be easily maintained within theabove range during the electrodeposition step and that ²²⁶Ra ions can bemore efficiently electrodeposited on a substrate, carboxylate salts arepreferable, mono- or divalent carboxylate salts are more preferable,acetate salts are yet more preferable, and ammonium acetate is stillmore preferable.

The pH buffer concentration in the electrodeposition solution may beappropriately selected according to the type of the pH buffer used;however, the pH buffer is preferably used so that the pH of theelectrodeposition solution is within the above range during theelectrodeposition step. The specific concentration is preferably 0.2 to1.0 mol/L and more preferably 0.2 to 0.8 mol/L. When the pH bufferconcentration is within this range, ²²⁶Ra ions can be more efficientlyelectrodeposited on a substrate.

In addition, from the viewpoints such as that ²²⁶Ra ions can be moreefficiently electrodeposited on a substrate, the ratio of using the acidand the pH buffer in the electrodeposition solution is preferably suchthat the electrodeposition solution is acidic at the start of theelectrodeposition step.

From the viewpoints such as that ²²⁶Ra ions can be more efficientlyelectrodeposited on a substrate, the amount of the pH buffer usedrelative to 0.02 mol/L of ²²⁶Ra ions is preferably 0.1 to 11.0 mol/L andmore preferably 0.2 to 11.0 mol/L.

<<²²⁶Ra Ions>>

²²⁶Ra ions are not particularly limited as long as ²²⁶Ra exists as ions,and, typically, a ²²⁶Ra salt or a solution containing this salt is used.

The ²²⁶Ra salt depends on the types of the acid and the alkalinesolution used in, for example, purification described below, andspecific examples thereof include nitrate salts, chloride salts,hydroxide salts, carboxylate salts, ammonium salts, and carbonate saltsof ²²⁶Ra. Although any of these salts can be used, since theelectrodeposition solution is preferably acidic at the start of theelectrodeposition step, nitrate salts, chloride salts, and carboxylatesalts are preferable from this viewpoint.

Since the ²²⁶Ra ions contained in the electrodeposition solution can beefficiently electrodeposited on a substrate by the present productionmethod, the amount of ²²⁶Ra ions in the electrodeposition solution maybe appropriately selected according to the desired amount of ²²⁶Ra to beelectrodeposited. The desired amount of ²²⁶Ra to be electrodeposited maybe determined by considering, for example, the radiation dozepermissible for the facility for producing ²²⁵Ac by using the obtained²²⁶Ra target.

The amount of ²²⁶Ra ions in the electrodeposition solution is, forexample, preferably 50 to 150 mg and more preferably 50 to 100 mg if thedesired amount of ²²⁶Ra to be electrodeposited is 50 mg.

Examples of the ²²⁶Ra ions that can be used include commerciallyavailable ²²⁶Ra or purified forms thereof, ²²⁶Ra ions obtained bypurifying a ²²⁶Ra salt-containing solution obtained by dissolving ²²⁶Raused as a radiation source in the medical or industrial field, and ²²⁶Raions obtained by purifying a ²²⁶Ra salt-containing solution obtained bydissolving a ²²⁶Ra target after production of ²²⁵Ac.

An example of the method for purifying a ²²⁶Ra salt-containing solutionis a method that includes an adsorption step (R1) of bringing a²²⁶Ra-containing solution (a) into contact with a carrier having afunction of selectively adsorbing divalent cations (hereinafter thiscarrier may be referred to as a “carrier (i)”) under an alkalinecondition so as to cause ²²⁶Ra ions to adsorb onto the carrier (i), andan elution step (R2) of causing the ²²⁶Ra ions to elute from the carrier(i) under an acidic condition. Performing this purification canconcentrate ²²⁶Ra ions and reduce impurities, and thus ²²⁶Ra ions can bemore efficiently electrodeposited on a substrate.

The carrier (i) is not particularly limited as long as the carrier formsa complex with metal ions under an alkaline condition and can elutemetal ions under an acidic condition, and the examples thereof includethose which have a divalent cation exchange group. Specific examples ofthe divalent cation exchange group include an iminodiacetic acid group,a polyamine group, and a methylglycan group, and the divalent cationexchange group is preferably an iminodiacetic acid group. The carrierthat has a divalent cation exchange group is not particularly limited aslong as the divalent cation exchange group is retained on a solid-phasecarrier such as a resin. A more preferable example is a styrenedivinylbenzene copolymer retaining an iminodiacetic acid group. Examplesof the commercially available products of the resin having animinodiacetic acid group include “Chelex” series produced by Bio-RadLaboratories, Inc., “DIAION” series produced by Mitsubishi ChemicalCorporation, and “Amberlite” series produced by The Dow ChemicalCompany, and a more specific example is “Chelex 100” produced by Bio-RadLaboratories, Inc. (particle diameter: 50 to 100 mesh, ion form: Naform, Fe form).

The carrier (i) may be packed in a tube and used. The tube is notparticularly limited as long as it can be packed the carrier (i) and hasflexibility, and is preferably a flexible tube made from rubber orresin, for example, and is more preferably a medical tube.

When such a tube is used, the length can be increased compared totypical glass columns, in other words, the theoretical plate number canbe increased; thus, the ²²⁶Ra ion adsorption efficiency can beincreased. Moreover, the carrier (i) through which a radioactivesubstance (²²⁶Ra-containing solution) has been passed can be kept packedin the tube and can be discarded easily without radioactivelycontaminating other equipment and devices, for example.

A specific example of the elution step (R2) is a method that involvespassing an inorganic acid through the carrier (i) to thereby elute ²²⁶Raions adsorbed on the carrier (i).

The inorganic acid may be any inorganic acid that can dissolve andionize the ²²⁶Ra component adsorbed on the carrier (i), and examplesthereof include hydrochloric acid and nitric acid.

Note that, from the viewpoints such as that ²²⁶Ra ions can beefficiently eluted from the carrier and that inorganic acid-derivedanions can be efficiently removed in a later step, the inorganic acidconcentration is preferably 0.1 to 12 mol/L, more preferably 0.3 to 5mol/L, yet more preferably 0.5 to 2 mol/L, and particularly preferably0.7 to 1.5 mol/L.

A step of washing the carrier (i) may be included between the step (R1)and the step (R2). Specifically, water is passed through the carrier(i). The proportion of the impurities can be further reduced by thiswashing.

The ²²⁶Ra ion-containing solution eluted in the elution step (R2) ispreferably subjected to an anion exchange step (R3) of passing thesolution through an anion exchange resin.

When anions (for example, chloride ions) derived from the inorganic acid(for example, hydrochloric acid) used in the elution step (R2) remainsin the solution, the ²²⁶Ra ion electrodeposition rate in theelectrodeposition step may be affected. Thus, the ²²⁶Ra ion-containingsolution eluted in the elution step (R2) is preferably treated in theanion exchange step (R3) since the anions derived from the inorganicacid can be exchanged to hydroxide ions and decreased, and the ²²⁶Ra ionelectrodeposition efficiency in the electrodeposition step can beimproved.

The anion exchange resin is not particularly limited as long as theanions (for example, chloride ions) derived from the inorganic acid canbe exchanged to hydroxide ions, and is preferably a strongly basic anionexchange resin and more preferably a resin having a quaternary ammoniumsalt. Examples of the commercially available products of such an anionexchange resin include “MONOSPHERE” series produced by The Dow ChemicalCompany, and “AG” series produced by Bio-Rad Laboratories, Inc., and amore specific example is “MONOSPHERE 550A” (particle diameter: 590±50mesh, ion form: OH form).

The anion exchange resin may be packed in a tube and used, as with thecarrier (i). Examples of the tube that can be used are the same as thosefor the above tube for packing the carrier (i).

<<Other Components>>

The electrodeposition solution may contain, if necessary, componentsthat have been used in, for example, electroplating as long as theeffects of the present invention are not impaired. One other componentor two or more other components may be used.

The electrodeposition solution preferably contains water, and the amountof water in the electrodeposition solution is, for example, preferably15 to 50 mL when the desired amount of ²²⁶Ra to be electrodeposited is50 mg.

It is also possible to use an alkali as appropriate from the viewpointof adjusting the pH of the electrodeposition solution, and examples ofthe alkali include sodium hydroxide, potassium hydroxide, and ammonia.

A specific example of the electrodeposition solution is anelectrodeposition solution that satisfies (a) to (d) below.

(a) contains ²²⁶Ra ions and a pH buffer.

(b) substantially free of alcohols.

(c) contains one acid or two or more acids, and these acids aremonovalent or divalent acids.

(d) contains carboxylate ions and preferably acetate ions.

Another specific example of the electrodeposition solution is anelectrodeposition solution that satisfies (a), (b), (e), and (f) below.

(a) contains ²²⁶Ra ions and a pH buffer.

(b) substantially free of alcohols.

(e) contains one acid or two or more acids.

(f) contains, as a pH buffer, a carboxylate salt, preferably amonocarboxylate or dicarboxylate salt, and more preferably an acetatesalt.

<Electrodeposition Step>

The electrodeposition step is not particularly limited as long as ²²⁶Rametal or a salt thereof can be electrodeposited on a substrate, and maybe the same step as an existing electroplating, for example, a methodthat involves inserting an anode and a cathode into theelectrodeposition solution and applying electrical current between theseelectrodes.

The anode is not particularly limited, and, for example, a platinumelectrode can be used. Substrates described below may be used as thecathode, for example.

<<Substrate>>

The substrate on which the ²²⁶Ra-containing substance is to beelectrodeposited is not particularly limited as long as the substratehas electrical conductivity; however, since the target to be obtained ispreferably irradiated with particles such as protons or γ ray by usingan accelerator such as a cyclotron or a linear accelerator, thesubstrate is preferably the one that is suitable for irradiation withsuch particles, and specifically preferably a metal substrate.

Examples of the metal used in the substrate include aluminum, copper,titanium, silver, gold, iron, nickel, niobium, and alloys containingthese metals (for example, phosphor bronze, brass, nickel silver,beryllium copper, Corson alloy, and stainless steel).

Alternatively, a substrate obtained by plating a conductive support withany of these metals may be used as the substrate.

From the viewpoints of, for example, reducing adverse effects on, forexample, facility used in irradiation with charged particles, photons,or neutrons, and suppressing contamination of a substrate-derived metalduring production of a radioactive isotope (RI) and contamination of asubstrate-derived metal during production of ²²⁶Ra ions from the targetafter production of the RI, a gold plate or a gold-plated plate ispreferably used as the substrate. Furthermore, by using a gold plate ora gold-plated plate as the substrate, ²²⁶Ra ions can be more efficientlyelectrodeposited on the substrate.

The shape of the substrate is not particularly limited, and may beappropriately selected according to the desired shape of the target;however, the substrate is preferably plate-shaped.

<<Electrodeposition Conditions>>

The power supply used for applying electrical current is notparticularly limited, and a DC power supply, an AC power supply, a pulsepower supply, or a PR pulse power supply, for example, can be used.Among these, a pulse power supply or a PR pulse power supply ispreferably used since such a power supply can easily evenlyelectrodeposit ²²⁶Ra ion-containing substance by improving ²²⁶Ra iondiffusion, can suppress generation of heat, and can performelectrodeposition by a small power supply, for example.

When a pulse power supply or a PR pulse power supply is used, the ONcurrent and the OFF current are preferably decreased, and the voltageduring electrodeposition is preferably decreased. In this case, forexample, the value of the ON current is preferably 0.1 to 0.3 A, and thevalue of the OFF current is preferably 0.0 to 0.2 A.

From the viewpoint of, for example, ease of separating bubbles generatedduring electrodeposition from the electrode, the ON time and the OFFtime are preferably both short. In this case, for example, the ON timeis preferably 10 to 90 msec, and the OFF time is preferably 10 to 90msec.

The electrodeposition time depends on the applied electrical current,and may be appropriately adjusted according to the desired amount of²²⁶Ra to be electrodeposited on a substrate; however, when a pulse powersupply or a PR pulse power supply is used, the electrodeposition time ispreferably 30 minutes or longer and more preferably 1 to 24 hours fromthe viewpoints such as that a target that can produce a desired amountof ²²⁵Ac can be easily obtained.

The temperature (temperature of the electrodeposition solution) duringthe electrodeposition step is not particularly limited, and, forexample, is about 10 to 80° C.

[Production method of ²²⁵Ac]

A production method of ²²⁵Ac according to one embodiment of the presentinvention includes an irradiating step of irradiating a ²²⁶Ra target,which has been produced by the present production method, with at leastone type of particles selected from charged particles, photons, andneutrons.

The particles are preferably protons, deuterons, α particles, or γ ray,and more preferably protons.

A specific example of the irradiating step is a step of acceleratingparticles, such as protons or γ ray, by using an accelerator, such as acyclotron or a linear accelerator and preferably a cyclotron, andirradiating the ²²⁶Ra target, which has been produced by the presentproduction method, with the accelerated particles.

Irradiating the ²²⁶Ra target with particles generates ²²⁵Ac via, in somecases, disintegration, for example. Purified ²²⁵Ac can be obtained byseparating and purifying ²²⁵Ac from the target that contains ²²⁵Acgenerated as such.

The method for separating and purifying ²²⁵Ac is not particularlylimited, and a known method can be employed; however, one example is amethod that involves dissolving the ²²⁵Ac-containing target by using,for example, an acid, adding an alkali to the obtained solution todeposit a ²²⁵Ac-containing salt, and separating and purifying the salt.

EXAMPLES

The present invention will now be further described through testexamples, but the present invention is not limited by these examples.

Note that the test that uses ²²⁶Ra cannot be easily conducted due to theissues associated with, for example, radioactivity; thus, in some of thetests described below, barium, which is considered to yield the sameresults as ²²⁶Ra, is used in testing. Radium is an element belonging tothe alkaline earth metal, and has properties similar to barium, which isalso an alkaline earth metal and has the closest mass to barium.Moreover, in the past, in extracting radium from pitch blend afteruranium extraction, the coprecipitation action with barium sulfate wasutilized; thus, it is known that radium and barium are very similar intheir properties.

Test Example 1

In a 0.05 mol/L aqueous hydrochloric acid solution, barium chloridedihydrate was dissolved to prepare an aqueous Ba hydrochloric acidsolution in a liquid amount of 2 mL and a Ba mass of 60 mg. Anelectrodeposition solution was prepared by mixing 14.4 mL of a 0.35mol/L aqueous ammonium acetate solution, 1.6 mL of a 0.1 mol/L aqueousnitric acid solution, and 2 mL of the prepared aqueous Ba hydrochloricacid solution. The pH of the electrodeposition solution measured with apH-test paper was 5 to 6. In preparing the aqueous solutions, ultrapurewater was used. The concentrations of the respective components in theelectrodeposition solution and the liquid amount of theelectrodeposition solution are shown in Table 1.

The prepared electrodeposition solution was placed in anelectrodeposition vessel, a platinum electrode was inserted thereto asthe anode, and a ϕ10 mm gold plate (thickness: 0.2 mm) was insertedthereto as the cathode (substrate). Next, pulse electrical current [acycle of applying 0.1 A electrical current for 10 msec and retaining thecurrent value of 0.0 A for 10 msec was continuously repeated (ONcurrent: 0.1 A, ON time: 10 msec, OFF current: 0.0 A, OFF time: 10msec)] was applied to these electrodes by using, as theelectrodeposition power supply, MPS-II-012010S10 (produced by ChiyodaElectronics Co., Ltd.) for 3.5 hours to electrodeposit Ba (Ba salt) onthe gold plate.

After applying the pulse electrical current for 3.5 hours, the goldplate was taken out and washed with ultrapure water, and the washed goldplate was dried at 100° C. for 1 hour.

The increase in mass after electrodeposition was calculated from thechange in mass between the gold plate after drying and the gold platebefore electrodeposition. Note that the “Average increase in mass afterelectrodeposition” described in the tables below is the average of theincrease in mass after the electrodeposition after performing the sametest multiple times. The results are shown in Table 1.

Test Examples 2 to 20

The average increase in mass after electrodeposition was calculated asin Test Example 1 except that the types and amounts (concentrations) ofthe respective components in the electrodeposition solution, the liquidamount, the substrate, and the electrodeposition time were changed asshown in Tables 1 and 2. The results are shown in Tables 1 and 2. Notethat the pH of the electrodeposition solutions obtained in these testexamples are all considered to fall within the range of 5 to 7.

TABLE 1 Average Electrodeposition solution increase AmmoniumHydrochloric in mass Ba acetate Nitric acid acid Liquid SubstrateElectrodeposition after Test mass concentration concentrationconcentration amount Diameter time electrodeposition name (mg) (mol/L)(mol/L) (mol/L) (mL) Type (mm) (hour) (mg) Test 60 0.280 0.009 0.006 18Gold ϕ10 3.5 21 Example plate 1 Test 60 0.400 0.009 0.006 18 Gold ϕ103.5 43.8 Example plate 2 Test 60 0.400 0.089 0.006 18 Gold ϕ10 3.5 45Example plate 3 Test 60 0.560 0.009 0.006 18 Gold ϕ10 3.5 29 Exampleplate 4 Test 60 0.800 0.009 0.006 18 Gold ϕ10 3.5 23 Example plate 5Test 60 0.444 — 0.006 18 Gold ϕ10 3.5 41 Example plate 6 Test 60 0.400 —0.011 18 Gold ϕ10 3.5 44 Example plate 7 Test 60 0.400 — 0.017 18 Goldϕ10 3.5 19 Example plate 8 Test 1.8 0.400 0.009 0.006 18 Gold ϕ10 3.5 2Example plate 9 Test 60 0.280 0.009 0.006 45 Gold ϕ20 3.5 40 Exampleplate 10 Test 60 0.400 0.009 0.006 45 Gold ϕ20 3.5 43.4 Example plate 11Test 60 0.560 0.009 0.006 45 Gold ϕ20 3.5 45 Example plate 12 Test 600.800 0.009 0.006 45 Gold ϕ20 3.5 42 Example plate 13 Test 4.5 0.4000.009 0.006 45 Gold ϕ20 3.5 4 Example plate 14 Test 225 0.400 0.0090.006 45 Gold ϕ20 3.5 12 Example plate 15 Test 315 0.400 0.009 0.006 45Gold ϕ20 3.5 8 Example plate 16 Test 450 0.400 0.009 0.006 45 Gold ϕ203.5 5 Example plate 17

TABLE 2 Average Electrodeposition solution increase Ammonium in mass Baacetate Acetic acid Liquid Electrodeposition after mass concentrationconcentration amount time electrodeposition Test name (mg) (mol/L)(mol/L) (mL) (hour) (mg) Test 34 0.45 0.05 25 3 35.4 Example 18 Test 340.45 0.07 25 3 28.1 Example 19 Test 34 0.45 0.1 25 3 24.8 Example 20

Test Examples 21 to 25

Electrodeposition solutions were prepared as in Test Example 1 exceptthat the amounts (concentrations) of the respective components in theelectrodeposition solution and the liquid amount were changed as shownin Table 3. Note that the pH of the electrodeposition solutions obtainedin these test examples are all considered to fall within the range of 5to 6.

The average increase in mass after electrodeposition was calculated asin Test Example 1 except that the obtained electrodeposition solutionswere used, a SUS plate (24×24 mm, thickness: 2 mm) was used as thesubstrate, and the conditions of the pulse electrical current and theelectrodeposition time were changed as shown in Table 3. The results areshown in Table 3.

TABLE 3 Average Electrodeposition solution increase AmmoniumElectrodeposition step in mass Ba acetate Nitric acid Hydrochloric acidLiquid ON current/ ON time/ Electrodeposition after mass concentrationconcentration concentration amount OFF current OFF time timeelectrodeposition Test name (mg) (mol/L) (mol/L) (mol/L) (mL) (A) (msec)(min) (mg) Test 40 0.28 0.0089 0.0056 18 0.2/0.1 10/90 20 6 Example 21Test 40 0.28 0.0089 0.0056 18 0.2/0.1 10/90 40 20 Example 22 Test 400.28 0.0089 0.0056 18 0.2/0.1 90/10 40 21 Example 23 Test 40 0.28 0.00890.0056 18 0.2/0.1 10/90 80 24 Example 24 Test 40 0.28 0.0089 0.0056 180.2/0.1 10/90 160 26 Example 25

Test Example 26

The average increase in mass after electrodeposition was calculated asin Test Example 1 except that the types and amounts (concentrations) ofthe respective components in the electrodeposition solution were changedas shown in Table 4 and a ϕ20 mm gold plate (thickness: 0.2 mm) was usedas the substrate. The results are shown in Table 4. The pH of theelectrodeposition solution obtained in Test Example 26 measured with apH-test paper was 6.

TABLE 4 Average increase Electrodeposition solution in mass AmmoniumHydrochloric Sodium after Ba acetate Nitric acid acid carbonate LiquidElectrodeposition electrode Test mass concentration concentrationconcentration concentration amount time position name (mg) (mol/L)(mol/L) (mol/L) (mol/L) (mL) (hour) (mg) Test 60 0.39 0.009 0.006 0.011118 6 60 Example 26

Test Example 27

An electrodeposition solution was prepared as in Test Example 1 exceptthat the amounts (concentrations) of the respective components in theelectrodeposition solution were changed as shown in Table 5.

The average increase in mass after electrodeposition was measured as inTest Example 1 except that the obtained electrodeposition solution wasused and that 0.1 A constant current was applied for 210 minutes byusing, as an electrodeposition power supply, MPS-II-012010S10 (producedby Chiyoda Electronics Co., Ltd.). The results are shown in Table 5. ThepH of the electrodeposition solution obtained in Test Example 27 isconsidered to be 6.

TABLE 5 Average Electrodeposition solution increase AmmoniumHydrochloric in mass Ba acetate Nitric acid acid LiquidElectrodeposition after Test mass concentration concentrationconcentration amount time electrodeposition name (mg) (mol/L) (mol/L)(mol/L) (mL) (min) (mg) Test 60 0.40 0.009 0.006 18 210 25 Example 27

Test Example 28

In a 0.05 mol/L aqueous hydrochloric acid solution, barium chloridedihydrate was dissolved to prepare an aqueous Ba hydrochloric acidsolution in a liquid amount of 1.1 mL and a Ba mass of 34 mg. Then 25 mLof an electrodeposition solution was prepared by mixing 12.5 mL of a 1mol/L aqueous acetic acid solution, 11.4 mL of a 1.1 mol/L ammoniawater, and 1.1 mL of the prepared aqueous Ba hydrochloric acid solution.In preparing the aqueous solutions, ultrapure water was used.

The increase in mass after electrodeposition was calculated as in TestExample 1 except that the obtained electrodeposition solution was used,that a ϕ20 mm gold plate (thickness: 0.2 mm) was used as the substrate,and that the electrodeposition time was changed to 3 hours. The increasein mass after electrodeposition was 31.3 mg.

Test Example 29

In a 0.05 mol/L aqueous hydrochloric acid solution, barium chloridedihydrate was dissolved to prepare an aqueous Ba hydrochloric acidsolution in a liquid amount of 1.1 mL and a Ba mass of 34 mg. Then 25 mLof an electrodeposition solution was prepared by mixing 15.625 mL of a0.4 mol/L aqueous succinic acid solution, 8.275 mL of a 1.5 mol/Lammonia water, and 1.1 mL of the prepared aqueous Ba hydrochloric acidsolution. In preparing the aqueous solutions, ultrapure water was used.

The increase in mass after electrodeposition was calculated as in TestExample 1 except that the obtained electrodeposition solution was used,that a ϕ20 mm gold plate (thickness: 0.2 mm) was used as the substrate,and that the electrodeposition time was changed to 3 hours. The increasein mass after electrodeposition was 18.4 mg.

Test Examples 30 to 32

The ²²⁶Ra target (size: conical shape with Φ10 mm and a thickness of 5mm, ²²⁶Ra mass: 0.4 to 0.6 mg) which had been irradiated with protonswas dissolved in 3 to 5 mL of 1 mol/L hydrochloric acid to recover a²²⁶Ra-containing solution (a-1).

Next, Chelex 100 (produced by Bio-Rad Laboratories, Inc., particlediameter: 50-100 mesh, ion form: Na form, amount used: 3 mL) convertedinto a NH₄ ₊ form was packed in a medical tube (EXTENSION TUBE producedby HAKKO CO., LTD., 3.2×4.4×500 mm (4 mL), MS-FL) having an innerdiameter of 3.2 mm, an outer diameter of 4.4 mm, and a length of 50 cm,50 to 80 mL of the obtained ²²⁶Ra-containing solution (a-1) (pH >9) waspassed through the tube at a flow rate of 1 to 2 mL/min, and the eluatewas discarded. Next, 10 mL of water was passed through Chelex 100 at aflow rate of 1 to 2 mL/min, and the eluate was also discarded.

Next, MONOSPHERE 550A (produced by The Dow Chemical Company, particlediameter: 590±50 mesh, ion form: OH form, amount used: 20 mL) wassequentially washed with hydrochloric acid, water, sodium hydroxide, andwater, packed in a medical tube (EXTENSION TUBE produced by HAKKO CO.,LTD., 3.2×4.4×500 mm (4 mL), MS-FL) having an inner diameter of 3.2 mm,an outer diameter of 4.4 mm, and a length of 200 cm, and connected tothe tube packed with Chelex 100 after 10 mL of water was passedtherethrough as indicated above.

From the Chelex 100-side of the thus connected tube, 10 mL of 1.0 mol/Lhydrochloric acid was passed at a flow rate of 1 mL/min, and then 8 ccof water was passed in a similar manner to obtain a Ra hydroxidesolution. The obtained solution was evaporated to dryness, and the driedproduct was dissolved in 1 mL of 0.1 mol/L hydrochloric acid. To thissolution, 2 mL of a 0.5 mol/L aqueous ammonium acetate solution wasmixed to prepare an electrodeposition solution. The pH of the obtainedelectrodeposition solution is considered to be about 5.

The ²²⁶Ra content in the obtained electrodeposition solution wasmeasured by radioactivity measurement with a germanium semiconductordetector produced by EURISYS MESURES. The results are shown in Table 6.

The ²²⁶Ra-containing substance was electrodeposited on a substrate byperforming the electrodeposition step as in Test Example 1 except thatthe prepared electrodeposition solution was used, that a ϕ10 mmgold-plated silver plate (a conical shape having a thickness of 5 mm)was used as the substrate, and that the electrodeposition time waschanged to 3 hours.

Since it is not easy to directly measure the ²²⁶Ra content in thesubstrate after the electrodeposition, the ²²⁶Ra content in theelectrodeposition solution after removing the substrate applying thepulse current for 3 hours was measured by radioactivity measurement witha germanium semiconductor detector produced by EURISYS MESURES, and thedifference in ²²⁶Ra content in the electrodeposition solution betweenbefore and after the electrodeposition was assumed to be the ²²⁶Racontent (amount of electrodeposited Ra) electrodeposited on thesubstrate. The results are shown in Table 6.

Note that Test Examples 30 to 32 involve the same testing except that adifferent target was used as the ²²⁶Ra target that had been irradiatedwith protons.

TABLE 6 Electrodeposition solution Ammonium Hydrochloric Amount ofAmount acetate acid Liquid Substrate Electrodeposition electrodepositedTest of Ra concentration concentration amount Diameter time Ra name(uCi) (mol/L) (mol/L) (mL) Type (mm) (hour) (uCi) Test 562 0.33 0.03 3Gold ϕ10 3 462 Example plated 30 Test 552 0.33 0.03 3 Gold ϕ10 3 477Example plated 31 Test 440 0.33 0.03 3 Gold ϕ10 3 330 Example plated 32

1. A production method of ²²⁵Ac comprising; a production step of a ²²⁶Ratarget comprising an electrodeposition step of electrodepositing a²²⁶Ra-containing substance on a substrate by using an electrodepositionsolution that contains ²²⁶Ra ions and a pH buffer, and an irradiatingstep of irradiating the ²²⁶Ra target with at least one selected fromcharged particles, photons, and neutrons.
 2. The production methodaccording to claim 1, wherein the electrodeposition solution issubstantially free of alcohols.
 3. The production method according toclaim 1, wherein: the electrodeposition solution comprises one acid ortwo or more acids, and the acids are monovalent or divalent acids. 4.The production method according to claim 1, wherein theelectrodeposition solution comprises carboxylate ions.
 5. The productionmethod according to claim 1, wherein the electrodeposition solution isacidic at the start of the electrodeposition step.
 6. The productionmethod according to claim 1, wherein the electrodeposition solution hasa pH of 4 to 9 during the electrodeposition step.
 7. The productionmethod according to claim 1, wherein the pH buffer is a monocarboxylateor dicarboxylate salt.